Polypropylene has taken on an important role as base material for the automotive industry. Today, it is widely used for exterior automotive applications, such as bumpers and side sills, as well as interior automotive applications, such as dashboards and interior trim. Polypropylene offers the advantages of having a low density, good mechanical strength, good heat resistance, good chemical resistance etc. as well as that of being a low-cost material in comparison to other suitable materials.
Originally the limited impact strength of polypropylene was a limiting factor in its use for automotive applications. This problem has, however, been overcome by blending polypropylene with elastomers. The elastomer component of the blend results in improved impact strength and an increased brittle temperature. Blends of this type are for example disclosed in document U.S. Pat. No. 6,797,779. Among a large number of possible polypropylenes metallocene-catalyzed polypropylenes are included as well.
Further, US-2004/0044107 discloses interior automotive parts comprising    (A) a crystalline homopolypropylene,    (B) a crystalline homopolypropylene and an ethylene-propylene copolymer rubber, said rubber having an intrinsic viscosity on 4.0 to 7.0 dl/g (decalin, 135° C.), ethylene content from 45% to 80% by mass and the rubber being present in at least 10% by mass of the homopolypropylene,    (C) a crystalline homopolypropylene and an ethylene-propylene copolymer rubber, said rubber having an intrinsic viscosity on 5.0 to 10 dl/g (decalin, 135° C.), ethylene content from 45% to 80% by mass and the rubber being present in at least 10% by mass of the homopolypropylene, and    (D) an ethylene/α-olefin rubber.
The resulting interior automotive parts are characterized by good molding properties and a good balance of physical properties, as well as good appearance, lower gloss and scratch resistance. The homopolypropylene (A) has a fraction soluble in boiling p-xylene, which suitably is 6.0% by mass or less, preferably 3.0% by mass or less, more preferably 0.1 to 2.0% by mass.
US-A-2007/0037914 discloses automotive parts formed from filled thermoplastic polyolefin compositions, wherein the filled thermoplastic compositions comprise a highly crystalline isotactic propylene homopolymer, an ethylene/α-olefin elastomeric impact modifier, and a reinforcing grade of a platy filler, for example talc. The invention aims at providing automotive parts that have excellent thermal and mechanical properties. The document makes not mention of emissions originating from such automotive parts.
In light of an overall reduction of emissions from cars the emission of volatile organic compounds (VOC) coming from polypropylene-based automotive parts has recently attracted special attention.
None of the above-cited documents addresses this problem it is noted in this respect that a low level of solubles e.g. in boiling p-xylene cannot be taken as an indication of low VOC emissions. It has to be kept in mind that the volatile organic compounds (VOC) are removed together with the p-xylene when the p-xylene is removed for the isolation of the solubles.
It is therefore an object of the present invention to provide polypropylene-based blends showing good mechanical properties and in particular good rigidity and impact strength while at the same time having low emissions of volatile organic compounds (VOC).